This invention relates to a heat exhanger, and more particularly to a heat exchanger suitably used as an intermediate heat exchanger for fast breeders.
The heat exchange between primary and secondary cooling systems in a fast breeder is carried out by an intermediate heat exchanger. The intermediate heat exchanger consists of two shells, i.e. outer and inner cylinders, and a plurality of heat transfer tubes provided in the inner cylinder. The upper and lower end portions of each of the heat transfer tubes are joined to tube sheets provided in the inner cylinder. A lower plenum is provided in the portion of the interior of the inner cylinder which is under the lower tube plate. A downcommer tube inserted into the inner cylinder is joined at its lower end with the lower plenum.
The sodium coolant in the primary cooling system, which is heated in the fast breeder, is supplied into the inner cylinder via the outer cylinder to flow downward along outer surfaces of the heat transfer tubes. On the other hand, the sodium coolant in the secondary cooling system, which is cooled by a steam generator, flows into the lower plenum through the downcommer tube. The sodium entering the lower plenum turns reversely, i.e. flows upward and into the heat transfer tubes. While the sodium in the secondary cooling system flows upward in the heat transfer tubes, it is subjected to heat exchange between itself and the sodium in the primary cooling system. Since the sodium in the secondary cooling system turns reversely in the lower plenum, the flow rate of the sodium passing through the heat transfer tubes is poorly distributed. Namely, the flow rate of the sodium flowing through such heat transfer tubes that are in a peripheral portion of the interior of the inner cylinder becomes higher. When the distribution of flow rates of the sodium flowing into the heat transfer tubes is not uniform, the distribution of temperature and velocity of flow of heat therein becomes uneven. This causes the performance of the heat exchanger to drop.
In order to settle the above problems, a heat exchanger disclosed in Japanese Patent Laid-Open No. 42690/1981 was proposed, in which a current-setting plate having a plurality of bores is provided in a lower plenum to divide the interior thereof into upper and lower regions with a downwardly extending tube inserted into the region below the current-setting plate. In an intermediate heat exhanger having such a current-setting plate, the distribution of flow rate of sodium introduced from a secondary cooling system into heat transfer tubes becomes uniform. However, in a transitional period of time, in which, for example, a nuclear reactor is interrupted, thermal impacts are repeatedly applied to the material forming the lower plenum.